Any ideea?

If you want to select multiple records from those listed in the list box,
the multi-select list box feature (see Access Help) will allow you to do
that.

If you want to select multiple copies of one record in a list box, you can
do that with a normal list box, and a text box to enter the number of copies
to <do whatever you want to do with>.

If you want to select multiple copies of multiple records, a List Box is not
the control you need. You could do that with a Subform Control and an
embedded Form.

On the other hand, if your design is such that you need to put multiple
copies of the same record into another table, that is likely an indication
that your design does not follow relational database design guidelines.  If
so, you should examine the design carefully, preferrably with the assistance
of and advice from, a relational database professional, because there's a
very high probability that correcting the design now will save you somewhere
between a little and a vast amount of time and energy in the future when you
work on this database.

Larry Linson
Microsoft Office Access MVP